Boreholes serve, in particular, for accessing and extracting crude oil or natural gas from underground deposits. These frequently extend over a very large surface area in the horizontal and vertical directions so that a plurality of boreholes is used for each of the individual deposits in order to recover the crude oil or the natural gas from the deposits.
The desired medium i.e. crude oil or natural gas in particular is situated in the deepest region of the borehole. This medium is extracted by means of pumps or other extracting devices. For the medium that has been removed in this way, further quantities of crude oil or natural gas flow in from the sides and from below. The upper surface of the fluids is called the interface depth.
A set of boundary conditions and in particular the precise location of the interface depth in the borehole is of interest for the purposes of operating the extraction process in a borehole. This depth is located some hundreds or more frequently several thousand metres deep and consequently can only be determined with difficulty as the gas above the liquid is not an ideal one but rather, it is a quite complicated and ever-changing mixture. A method and a device for detecting the location of the interface depth in a borehole are known from EP 2 169 179 B1 and U.S. Pat. No. 8,902,704. These devices and the method performed thereby are employed in many places with great success. In the event of the interface depth sinking below a certain amount, the borehole would then dry-out and naturally, this is something which is not desired. On the other hand, there is a desire to extract as large a quantity of crude oil as possible from a borehole. Due to the fact that the interface depth can now be determined, it is possible to achieve a significant improvement here.
The improved knowledge of the location of the interface depth in a borehole is also valuable for reason that the places where the process for extracting crude oil or natural gas takes place often lie in regions which are unfavourable for climatic or other reasons. As a rule, there are no operating personnel in the vicinity of the place where the extraction process is taking place, i.e. the borehole, and it is also frequently desired to let the boreholes work automatically for as long a period as possible. Consequently, a check on the situation at the borehole is only carried out at very long time intervals. This also applies for a new installation or control system at the borehole.
Each visit to the borehole is thus associated with relatively high costs and the expense required for the visit and transportation and it is desirable for the automated process to be continued insofar as possible.